Lizzie thinks the Ververs, her next-door neighbors, are more or less a perfect family. Their daughter Evie is her best friend. And then one day Evie disappears. Everyone is sure she's been abducted, and Lizzie uses her special knowledge to do some investigating of her own. She's not above planting evidence if she has to.
There's not much mystery in the usual sense here. Everyone seems to know who the culprit is. The question is what really happened. And why. Those are the things that bother Lizzie, too, and she gradually finds out the answers. Maybe not all of them, but the reader again understands a little bit more than she does.
Excellent and evocative writing as usual from Abbott. Check it out.